I have worked with each one (and a couple of them are running now at home for home monitoring). The real interesting one is the last in the list and is the least expensive given the capabilities - it comes loaded with Ubuntu 12.04 (full version - not stripped down) and works straight out of the box. All of the others I had to configure with an OS installed on a micro-SD card - the exception was the Gumstix which has a version of Angstrom Linux running on it :)

Any one of the above devices will perform well as a web server - all are running ARM based processors - the PandaBoard is running a dual-core processor and is very responsive. All use less than around 4 watts of power so are definitely low power devices.

Yeah, sadly blades make them useless in the SMB and nearly useless elsewhere. There is a big gap between "a low power server" and a "bank of low power clusters." What they are creating, like Moonshot, is a physical cloud architecture.